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I first noticed some funny things going on about 4 weeks ago wile driving my 79 truck would start to overheat after i first started it in the morning the needle on the gouge would go to 7/8s hot or so and then all of a sudden drop to normal and everything would be ok as long as i drove and it stayed fairly warm in between driving, but after it was parked for a wile and had cooled it would do the same thing again jump up to hot and then drop to normal, i flushed the radiator . purchased a new radiator cap . and i found a leak in the radiator that i fixed by removing the radiator and soldering the tiny hole shut, this worked and now my radiator has no leaks . but it still did the getting hot and then dropping to normal thing, well a couple of days ago when i left my house in the morning the gauge went to almost red hot and then stayed there and wile the gauge showed hot my heater started to blow cold air only and does the same thing every time i drive it now . I thought is was my thermostat and today i got a new one and installed it this evening wile the thermostat was off i filled the cooling system with antifreeze mix 50 50 to just below the open hole so it would not spill wile i placed the the new thermostat in place and bolted it down , this way i made sure the engine was full of coolant i after installing the thermostat hose then topped of the radiator and took a test drive , at first i thought i had fixed the problem because as the engine was heating up i could feel hot air from my heater but when it reached the normal mark on the temperature gauge allofasudden my heater started blowing cold air again like there was no hot water flowing threw it and my temperature shot up to almost read hot , i had my overflow coolant reservoir empty and i noticed it filled with hot radiator fluid but my truck never showed signs of completely boiling over it never shot out any steam but stayed very hot until i shut it off . So fixing the thermostat did nothing at all . what could cause my truck to overheat but not blow hot air from the heater? I mean not even slightly warm air its cold here in Washington and its blowing ice cold air . Also after letting it cool i looked in the radiator and i still had fluid that i could see, it was missing some that went into the radiator overflow tank but it still had enough fluid in it is should have cooled the truck just fine , i also have no leaks from the waterpump and the waterpump is making no noise at all everything seems normal the belt is tight and not slipping. Anyone has any ideas

Does it do exactly the same after you changed the thermostat? There is a flow issue here that is going to get a head gasket pretty quick. If it warms up and blows hot air while the t-stat is closed, then cools off after opening, radiator could be clogged. if it is fine, fins could be worn off impeller in water pump.(while my expertise in the stock motors consists of pulling them out for a v8 swap, I HAVE seen this on various SBC's over the years...) First part of your post sounds exactly like a bad t -stat, and I have seen new ones crap out too. Try putting one you plan on using, (or the old one if you haven't pitched it) in a pan of water on the stove and using a thermometer to see if it is opening correctly and at the proper temperature. Good luck...

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Thanks for the reply good thoughts . Last night after posting this i drained my antifreeze and disconnected my hoses coming from the heatercore and i backflushed the heatercore and i also flushed the hose going back to my radiator i just got up and will do it again before i even thought about removing my newly installed thermostat because that did not make any difference and do a test drive like that also without the thermostat installed i should be able to see a good water flow inside the radiator with the cap off if the waterpump is working and nothing is clogged. I would think with the thermostat removed the fluid in the radiator should circulate instantly as soon as the truck is started. Right? I also will do the thermostat test in a pan of water i got a food thermometer where i can test the water temperature to see when it opens .

It also can mean that the head gasket is leaking combustion gases into the cooling system. This will cause air pockets and the water pump can't pump air, only water. It will become totaly erratic and get worse with time untill the head gasket is replaced.

If there is an air pocket in the system you will not get coolant flow.It basically vapor locks the system.I would run it with the radiator cap off in your driveway.Also if you see bubbles you might have a blown head gasket.

Well as soon as i got up this morning i flushed my system again i did it from the front of the engine to the back of the engine and from the ingoing side of the heatercore to the outgoing side backward and forward and there was absolutely no evidence of any plugs . Then i test drove it and it was wierd my temperature gauge stayed on cold even know i was driving way past the distance it normally takes for the truck to warm up i finally lost my nerve and turned around and drove back home when i turned in to the driveway my gauge still showed a cold engine and cold air was blowing from my heater. I then took out my new thermostat that i just installed last night and refilled the truck with water . As soon as i eat my lunch i will test drive it now without the thermostat installed just to see if that makes any difference and yes i will run it first in the driveway for a wile with the radiator cap removed to let any air in the system escape and to also look into the radiator to see if the water is actually moving it i think should be instantly and always circulating now because i just removed the thermostat. Am i right on that

pull the plugs, and see if any have white deposits around the tip. if you do, you may have blown the headgasket into that cylinder. also, go to a place like checker or vatozone and borrow (or buy if you like) a radiator cap/ radiator pressure tester. It is an invaluable tool.

pressurize the system and see if you see any other leaks or if the pressure drops after a bit, you have air in the system/ or air getting INTO the system. that's ususally a sign of a bad head gasket, or pump gasket.

do the plugs first and then get back to us.

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Ok ill pull the plugs good idea.

I had that exact problem and it turned out to be the thermostat.

If you try it without one, as you're planning to do, the truck may never reach temperature but I think you'll see the problem go away.

You have to be really careful when installing the thermostat, because the replacements that you get at the auto parts are just slightly larger that the housing, and don't fit correctly into the rim.

I ended up putting a thermostat for a sbc in and it works better than any that were supposedly for the Luv. Of course, at the auto parts, they don't know what to do if you ask for a small block Chevy thermostat, so I told them it was for a 1970 Nova. That worked. I had to file the edge of the thermostat just a hair to get it to seat properly but my temperature gauge sits like a rock at 180 degrees now.

One final thought: The temperature sender is under the intake manifold, with a wire coming off it to the right fender. Until I modified mine, I would accidentally knock it loose all the time. This will cause the temp gauge to read stone cold. I recommend you check that it didn't come loose.

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